1. Field of the Invention
The present invention relates to radiation imaging, more specifically to an X-ray source and an X-ray photographing apparatus each including a transmission type target electrode.
2. Description of the Related Art
A thermionic source is conventionally used as an electron source of an X-ray generating apparatus. In an X-ray generating apparatus that uses a thermionic source, part of thermally emitted electrons (thermions) emitted from a filament heated to high temperature are formed into an electron flux of a predetermined shape through a Wehnelt electrode, an extraction electrode, an accelerating electrode, and a lens electrode; and the electron flux is accelerated to have high energy. A target electrode including a metal such as tungsten is irradiated with the electron flux, thereby generating X-rays. As the thermionic source, there is known a small-sized thermionic source such as an impregnated hot-cathode electron emission element that is also known as an electron source of a cathode-ray tube.
It is to be noted, however, that out of entire energy that the electron flux possesses, only about 1% or less of the energy is converted into X-rays while the remainder becomes heat. Since the target electrode resides within a vacuum chamber, most of the heat is radiated as radiant heat. If heat radiation is not effectively evacuated from the vacuum chamber, then temperature of the target electrode rises and the target electrode often melts. Because of this, the conventional X-ray generating apparatus is designed to reduce a quantity of electrons colliding on the target electrode per unit area and to adjust the energy applied to the target electrode per unit area. To reduce the quantity of electrons per unit area, it is effective to increase an electron irradiation area.
On the other hand, a portion of the target electrode against which electrons collide serves as an X-ray generation unit. The X-ray generation unit cannot be excessively enlarged since a size of the X-ray generation unit has an effect on resolution of an X-ray detector.
To realize both a reduction in the quantity of electrons per unit area and an improvement in the resolution, a technique for tilting a surface of the target electrode with respect to an electron irradiation direction and a technique for providing very small irregularities on the surface of the target electrode have been proposed. However, when the technique for tilting the surface of the target electrode and that for providing very small irregularities on the surface of the target electrode are adopted, the X-ray generating apparatus effects different focal sizes according to X-ray extraction directions and the resolution tends to deteriorate. This is because an area of a region irradiated with an electron-beam geometrically changes depending on the X-ray extraction direction. Since the deterioration in the resolution is possible, a user performing X-ray photography needs to check a tilt direction of an X-ray target and make settings to arrange the X-ray target in consideration of regions where the focal size is apparently small when X-ray photographing requires high resolution. In other words, it is a burden on the user to make complicated preparations for the X-ray photography that requires high resolution when the conventional X-ray generating apparatus is used.